1. Field of the Invention
The present invention relates to the color balancing of photosensitive materials, in general, and to an optical filter system for automatically effecting a color balanced exposure of photosensitive material within a photographic camera, in particular.
2. Description of the Prior Art
The color balance of an image formed in a photosensitive material is dependent upon several factors. One factor is the color balance of the photosensitive material itself. The continuous manufacture of large quantities of photosensitive materials over extended periods of time, especially materials of the self-developing type, requires fairly complex processes that are relatively difficult to control. One consequence of employing such complex processes is an occasional unwanted shift in the color balance from a nominal or desired color balance, a shift that produces an excessive level of one particular color in an image subsequently formed in such materials. A more detailed explanation of this problem is described in U.S. Pat. No. 4,329,411 to Land.
Image color balance is also affected by the color temperature of scene illumination. The color temperature may produce a concentration of light frequencies at the higher or lower energy portions of the spectrum. An outdoor exposure of a scene, for example, may include a substantial area of blue sky having a relatively high color temperature with the scene light being predominately composed of relatively high frequency radiation at the blue end of the spectrum.
A color balance shift will also occur in an image formed in photosensitive materials of the self-developing type if the temperature of a processing fluid forming a portion of a film unit employing such materials is raised or lowered beyond certain temperature limits during material processing.
Optical filters have been employed in the past for balancing the color of an image formed in photosensitive materials located within a photographic camera. In commonly assigned U.S. Pat. No. 4,736,215 to Hudspeth et al, for example, a film cassette is provided with indicia or machine readable information on an external surface thereof corresponding to one or more film variables of a film unit enclosed within the film cassette. A camera into which the film casette is insertable is provided with three optical filters for controlling photosensitive material color balance, each of which is selectively movable into the optical axis of a taking lens of the photographic camera under control of signals developed by reading means within the camera responsive to the film cassette indicia. A disadvantage of this optical filter color balancing system is that it only compensates for color imbalance factors of the photosensitive material itself and not for color imbalance generating factors external of the film or film cassette.
In commonly assigned U.S. Pat. No. 3,468,228 to Rogers, an automatic shutter mechanism for a photographic camera is disclosed which incorporates a selection of color balancing filters. The filters compensate for color balance shifts produced by scene illumination color temperature. The filters are simultaneously positioned over both the photosensor of an exposure timing system and the taking aperture of a shutter for a time interval that is inversely related to the intensity of the light transmitted by the compensating filter. A disadvantage of this color balancing optical filter system is that it only compensates for color imbalance caused by scene illumination color temperature and not for color imbalance factors caused by the photosensitive material or other color imbalance producing factors external of the photosensitive material.